1. Field of the Invention
The present invention relates to surface-emitting laser elements, surface-emitting laser arrays, optical scanning devices, and image forming apparatuses, and more particularly to a surface-emitting laser element, a surface-emitting laser array, an optical scanning device including the surface-emitting laser element or the surface-emitting laser array, and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
A vertical cavity surface-emitting laser element emits light in a direction perpendicular to a substrate. Compared to an edge emitting type semiconductor laser that emits light in a direction parallel to a substrate, the vertical cavity surface-emitting laser element is low price, low power consumption, appropriate for two-dimensional device, and high performance. Therefore, in recent years and continuing, vertical cavity surface-emitting laser elements are attracting attention.
The application areas of vertical cavity surface-emitting laser elements include a light source in an optical wiring system in a printer (oscillation wavelength band: 780 nm), a light source used for writing light in an optical disk device (oscillation wavelength band: 780 nm, 850 nm), and a light source in an optical transmission such as a LAN (Local Area Network) using optical fiber (oscillation wavelength band: 780 nm, 850 nm, 1.3 μm, 1.5 μm). Other expected application areas of vertical cavity surface-emitting laser elements include a light source for optical transmission between boards, inside a board, between chips in a LSI (Large Scale Integrated circuit), and inside chips in an LSI.
In these application areas, the light emitted from the surface-emitting laser element (hereinafter, also referred to as “emission light”) is preferably in a single transverse mode and is preferably high output. Particularly, there is demand for light of fundamental transverse-mode oscillation and high output. Accordingly, higher-order transverse mode needs to be prevented, and various attempts have been made to achieve this.
For example, patent document 1 discloses a surface emission semiconductor laser element including the following elements. A layer structure of a semiconductor material, where an emission layer is sandwiched between upper and lower reflector layer structures, is formed on a substrate. An upper electrode having a ring shape in a planar view is located above the upper reflector layer structure. There is an opening on the inside of the electrode. Part of the surface of the opening is coated by a layer that is transparent with respect to the oscillation wavelength of the oscillation laser beam.
Patent document 2 discloses a surface emitting semiconductor laser including the following elements. An active layer has a light emitting center area. A pair of multilayer film reflection mirrors is provided, with the active layer sandwiched therebetween. The multilayer film reflection mirrors have light emitting regions on one side thereof. An electrode has an opening corresponding to the light emitting regions. An insulating film is provided corresponding to the light emitting regions. The insulating film includes a peripheral area surrounding a center area that corresponds to light emission center areas of the light emitting regions. The insulating film is configured such that the reflection ratio of the peripheral area is lower than that of the center area.
Patent document 3 discloses a surface-emitting semiconductor laser including the following elements. A laser structure is formed by laminating the following layers on a substrate. Specifically, a first multilayer film reflection mirror, an active layer having a light emitting center area, a second multilayer film reflection mirror, and a transverse mode adjusting layer are laminated on the substrate in the stated order. Either one of the first multilayer film reflection mirror or the second multilayer film reflection mirror has a current injection area having a quadrangular shape, whose intersection of diagonals corresponds the light emitting center area. The second multilayer film reflection mirror includes a light emission opening provided in an area corresponding to one of the diagonals of the current injection area, and a pair of groove parts having the light emission opening provided therebetween. The transverse mode adjusting layer is provided so as to correspond to the light emission opening. A peripheral area of the light emission opening, which is an area excluding a center area that corresponds to the light emitting center area, has a reflection ratio that is lower than that of the center area.
Patent document 4 discloses a surface-emitting semiconductor laser element including a first multilayer reflection film, an active layer formed on the first multilayer reflection film, and a second multilayer reflection film formed on the active layer. At least one of the first multilayer reflection film or the second multilayer reflection film is positioned in at least part of an area corresponding to the active layer, and has a first region with a thickness of substantially λ/4n (λ: oscillation wavelength, n: refractive index) and a second region disposed on a region other than the first region and that has a thickness other than substantially λ/4n.
However, in the surface-emitting semiconductor laser disclosed in patent document 3, if the interval between grooves is made narrower than the current narrowed region for specifying the light deflection direction, the current passing region is substantially narrowed. Consequently, the electric resistance of the laser element may increase, or the current density may increase, which shortens the service life of the laser element.
Furthermore, in the surface-emitting semiconductor laser element disclosed in patent document 4, the manufacturing method needs to be performed as follows. That is, crystal growth is performed up to the layer adjacent to the active layer. Then, the crystal growth is interrupted, and resist patterning and film etching is performed. Subsequently, crystal growth needs to be performed once again. In this case, when crystal growth is performed once again, the condition of the surface of the film that has undergone etching affects the crystal growth. Consequently, variations are caused in properties of the laser element and properties of controlling the transverse mode. Accordingly, this method is inappropriate for high-volume production of devices.
Inventors of the present invention performed various experiments and considerations with respect to a structure in which an optically transparent film is formed on a surface, and the transverse mode is controlled by making the center part of a light emitting area have a different reflection ratio from that of a peripheral part of the light emitting area. With such a structure, it has been found that the shape of the low reflection ratio part, particularly variations in the size of the low reflection ratio part, significantly affects the function for preventing the higher-order transverse mode, which may decrease the production yield.
Patent Document 1: Japanese Laid-Open Patent Application No. 2001-156395
Patent Document 2: Japanese Laid-Open Patent Application No. 2006-210429
Patent Document 3: Japanese Laid-Open Patent Application No. 2007-201398
Patent Document 4: Japanese Laid-Open Patent Application No. 2004-289033